How to Catch Exceptions Raised in Child Threads in Python?

Catching a Thread's Exception in the Caller Thread

In Python, multithreaded programming presents challenges in handling exceptions that occur in child threads. When exceptions arise in a child thread, they remain confined to that thread, making it difficult for the caller thread to capture and process them.

Core Issue and Rationale

The main issue lies in the immediacy of the thread_obj.start() method. This method initiates the child thread's execution in its own context and stack. Consequently, any exceptions that occur within the child thread are not propagated back to the caller thread.

Overcoming the Issue

To address this problem, one effective technique involves employing message passing. In this approach, a communication channel is established between the child and caller threads, enabling the child thread to convey exception information to its parent.

Implementation

Below is an example that demonstrates how to capture child thread exceptions and display them in the caller thread:

import sys
import threading
import queue

class ExcThread(threading.Thread):

    def __init__(self, bucket):
        threading.Thread.__init__(self)
        self.bucket = bucket

    def run(self):
        try:
            raise Exception('An error occured here.')
        except Exception:
            self.bucket.put(sys.exc_info())


def main():
    bucket = queue.Queue()
    thread_obj = ExcThread(bucket)
    thread_obj.start()

    while True:
        try:
            exc = bucket.get(block=False)
        except queue.Empty:
            pass
        else:
            exc_type, exc_obj, exc_trace = exc
            # deal with the exception
            print exc_type, exc_obj
            print exc_trace

        thread_obj.join(0.1)
        if thread_obj.isAlive():
            continue
        else:
            break


if __name__ == '__main__':
    main()

Explanation

In this example, the ExcThread class acts as the child thread responsible for generating an exception. The main() function serves as the caller thread. A communication channel is established using the queue.Queue() object (called bucket in the code).

Within the run() method of the ExcThread, any exception that occurs is placed into the bucket. The caller thread continuously checks the bucket in a loop. When an exception is available in the bucket, it is retrieved and the details of the exception (type, object, and traceback) are printed out.

This approach effectively bridges the communication gap between the child thread and caller thread, enabling the latter to catch and handle exceptions generated in the former.

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